1. Field of the Invention
This invention relates to prosthetic devises in general and, more particularly, it relates to a knee unit for a prosthetic leg.
2. Description of the Prior Art
One of the primary problems in providing a satisfactory above-knee prosthetic leg involves controlling of the swing phase of the walking cycle, including the terminal impact and the extension cycle regarding its speed and intensity. The terminal impact is known to be the resultant force generated during the swinging motion of a lower limb and an upper limb with respect to each other, when the prosthetic leg reaches the end of its swing phase, prior to achieving a substantially straight position. When humans advance in the walking cycle, a leg bends at the knee and is lifted above the ground. In order to be supported by the ground again, the leg has to be fully extended. Prosthetic legs operate in a similar manner. Furthermore, if the motion of the prosthetic lower limb caused by the forces of the terminal impact is not decelerated, a very revealing and often embarrassing noise is produced. To the discomfort of a patient, this makes the presence of the prosthesis very obvious, causing other people to be aware that the patient has an artificial leg. The terminal impact also negatively affects the structural elements of the prosthetic device itself, often leading to their damage. All of these emphasize the necessity of minimizing the effect of the terminal impact.
Another negative aspect of the prior art prosthetic devices is the artificially high rate at which the leg advances during the swing phase to its complete straight position. This is quite different from a normal leg which slows down at the end of the travel. The swing phase of the walking cycle causes the leg to accelerate in its motion from being bent to being straight. At the beginning of the swing phase, the normal leg moves rapidly, so that it can be situated underneath the patient's body. At the end of the swing phase the leg typically slows down. In the prosthetic devices, in the process of reaching the stop in full extension, an undesirable momentum of forces develops which has to be slowed down or minimized. If such momentum is not minimized, one part of the leg will crash against the other part of the leg. The prior art prosthetic knee devices typically utilize hydraulic-based systems for their operations. In view of the presence of the liquid mediums (such as oil, etc.), which are difficult to compress, the prosthetic devices move at a substantially constant rate during the entire walking cycle. Therefore, the highly desirable deceleration of the movement of the prosthetic leg at the end of the travel is not typically provided. Thus, there is an obvious need for a prosthetic device capable of regulating and decelerating its motion during the swing phase before it reaches a substantially straight position.
Various hydraulically operated knee units for above-knee prosthetic legs are known in the prior art. However, even sophisticated knee units, as disclosed, for example, by the inventor's own U.S. Pat. No. 5,779,735 do not provide prosthetic devices capable of efficiently avoiding the terminal impact, and slowing down the motion of the limbs prior to the leg reaching substantially straight position. All of these problems are substantially alleviated in the prosthetic knee device of the invention which utilizes the compressible nature of gas or air for negating the undesirable forces. The prosthetic device of the invention closely simulates the movement of human limbs, without being subjected to the results of the terminal impact. It moves rapidly in the initial stages of extension, and then slows down before reaching the stop in full extension.